Doped silicon nitride article

ABSTRACT

A process for making a homogeneous yttria-alumina doped silicon nitride article is described. A uniform coating of yttria and alumina is applied to the surface of silicon nitride particles by a chemical application of the nitrates of yttrium and aluminum followed by drying and a subsequent conversion of the nitrates to the corresponding oxides. The resulting powder is formed into an article and pressureless sintered to a density greater than 98% of theoretical at temperatures less than 1700° C.

This is a continuation of co-pending application Ser. No. 761,457, filed on Aug. 1, 1985, now abandoned.

FIELD OF THE INVENTION

This invention relates to a method of making a ceramic article. More particularly, this invention relates to a method of making a silicon nitride article having homogeneously dispersed sintering additives.

BACKGROUND OF THE INVENTION

A major problem in ceramic processing is the capability of manufacturing reproducible and reliable ceramic materials having the required properties suitable for high technology applications. This lack of reproducibility of material properties lies in the inability to control the development of specified microstructure, which for sintered materials depend on the characteristics of the starting powder, the green compact microstructure, and the sintering and coarsening processes. Although much significant work has been performed on the process which occurs during firing many researchers believe that the microstructure developed during sintering is determined to a large extend by the powder characteristics and the green microstructure. Therefore, the key factor to stronger and more reliable ceramics lies in new processing techniques that would result in fewer of the flaws and voids that lead to failure.

SUMMARY OF THE INVENTION

In accordance with the present invention, a new and improved process for making a silicon nitride article having homogeneously dispersed sintering additives comprises the following steps:

Step 1--Silicon nitride particles are dispersed in a dispersion reagent to form a silicon nitride slurry.

Step 2--A solution of sintering additive is added to the silicon nitride slurry to form a homogeneous slurry having a pH less than 7.

Step 3--The product from step 2 is dried to form a dried powder which has an essentially uniform coating of the sintering additives on the silicon nitride particles.

Step 4--The sintering additives of the product from step 3 are converted to their corresponding oxides.

Step 5--The product from step 4 is formed into a silicon nitride article.

Step 6--The product from step 5 is pressureless sintered at a temperature less than 1700° C. to form a densified silicon nitride article which has a density greater than 98 percent of theoretical density.

DETAILED DESCRIPTION OF THE INVENTION

Two Si₃ N₄ based formulations, one having 6 w/o Y₂ O₃ as a sintering aid, designated as PY₆, and the other having and 6 w/o Y₂ O₃ and 2 w/o Al₂ O₃ as sintering aids and designated as AY6, were used to illustrate the advantages of the new and improved processes of the present invention. The processing technique can be applied to other systems in which oxides are used as sintering additives.

One Powder Processing method is based on the use of inorganic salts of the sintering aids.

This method is based on the addition of aqueous solutions of sintering additives such as aluminum nitrate and yttrium nitrate for AY6 formulations or yttrium nitrate for PY6 formulations to a slurry of a dispersed Si₃ N₄ powder. Other sintering additives such as the soluble salts of magnesium, lanthanum, cerium, hafnium, zirconium, and the rare earths can be used.

In general, premilled Si₃ N₄ powders are dispersed in high dielectric constant solutions by the use of an ultrasonic liquid processor and/or wet-milling techniques. The dispersion agents can be low molecular weight alcohols, acetone, weak alkali media such as NH₄ OH, or weak acidic media such as 1 w/o ammonium citrate solution. The resulting dispersion containing a nitrate solution of sintering additives has a pH less than 7.0 and a slip density of about 1.3 g/l with a solids loading from about 30 to 35 volume percent solids loading. This dispersion is then spray dried or pan dried to produce loosely agglomerated powders, spherical in the case of spray dried powders, followed by calcination at 400°-500° C. under N₂ /air to decompose nitrates to their corresponding oxides. The powder so prepared has a surface area which is 1.5 to 3.0 times higher than those of the conventional ball milled material. A billet, prepared by isostatic pressing, slip-cast forming techniques, can be sintered to about 98.0% of theoretical density at a temperature less than 1700° C. and at an ambient pressure with improved strength and reliability. Besides nitrates, other inorganic salts can also be applied to this method such as acetates, oxalates, and lactates.

Another method utilizing Metal Alkoxides Precursors is based on the polymerization (i.e. hydrolysis and polycondensation) reactions of aluminum alkoxides (aluminum ethoxide, aluminum isopropoxide or aluminum tetrabutoxide) with yttrium isopropoxide in the presence of a limited amount of H₂ O to produce an inorganic gel probably with networks like Si--O--Y--O--Si--O--Al for the AY6 formulation. The advantage of this method is that all the desired multicomponent oxides can be produced with well defined stoichiometry after hydrolysis and polycondensation. In general, the glassy phase prepared from this method can be formed at a much lower processing temperature.

For example:

The alkoxide precursors are added to a well dispersed alcohol solution of Si₃ N₄.

An inorganic polymer is formed on the Si₃ N₄ particles by the hydrolysis of the alkoxides. The Si₃ N₄ is homogeneously coated with oxides of the sintering additives after a calcination step which converts the inorganic polymers to their corresponding oxides.

The calcined powder is then formed into pellets or billets and pressureless sintered to form densified articles.

The results of sinterability studies of the processed AY6 materials formed as pellets and as billets are tabulated in Table I and Table II, respectively. Results indicate that the powders processed by the new and improved process are more reactive, and that the pellets can be sintered to theoretical density at 1710° C. under 200 psi N₂ regardless of the density of the green pellet. The superiority of the powder is evident at lower sintering temperature and pressure. Pellets were sintered to greater than 99.0% at 1750° C. under 60 psi N₂ pressure as compared to 97.5% for the control lot which used conventional ball milled powders. In general, this new chemical processing of sintering additives of Si₃ N₄ provides technical potential in terms of sinterability at reduced temperature, pressure and in terms of grain size control of the sintered compact. The mechanical properties of these billets, AY6, PY6, sintered at new sintering schedules have been evaluated from ambient to 1200° C. The results are comparable to those data obtained from billets processed from conventional ball milled powders and sintered at 1850°, 200 psi N₂ overpressure for AY6 material and 1900°, 200 psi N₂ overpressure for PY6, but with higher reliability.

Examples of the Experimental Procedure:

General: The Si₃ N₄ powder used in this invention was produced by GTE Products Corporation designated as SN502. The SN502 powder is a high purity material produced by the reaction between SiCl₄ and NH₃ (g). The powder is a crystalline material consisting of equal to or less than 92% of alpha-Si₃ N₄, 7% beta-Si₃ N₄, and less than 1% free Si. The surface area is about 2.5-4.0 m² /g. Yttrium nitrate (99.9% purity) and reagent grade aluminum nitrate purchased from AESAR Johnson Mathy, Inc./Aldrich were used to prepare sintering additives.

The raw SN502 powder was usually milled with 4.0 weight percent stearic acid for 48 hrs. to 72 hrs. with Si₃ N₄ milling media in a Scott Murray mill. The milled powder was then sifted through a 50 mesh screen followed by the removal of stearic acid at 500° C. prior to chemical processing. The particle distribution of the milled SN502 is from 0.1 to 12 microns with 50 weight percent less than 0.6 microns, and 85 weight percent less than 2.0 microns. After processing, the typical particle size distribution is from 0.8 microns to 20 microns with 50 weight percent less than 5 microns 85 weight percent less than 10 microns.

Preparation of Sintering Aid Formulations:

A formulation of silicon nitride having 6 w/o Y₂ O₃ and 2 w/o Al₂ O₃ is designated as AY6 and silicon nitride having 6 w/o Y₂ O₃ is designated as PY6.

In the case of PY6, 37.0 g of SN502 silicon nitride powder prepared as described above was combined with 100 ml 1 w/o ammonium citrate solution, 27.5 ml of yttrium nitrate solution at a concentration equivalent to 86.6 g/l of Y₂ O₃, and 72 ml H₂ O. The mixture was ultrasonically dispersed using a Model W375 ultrasonic liquid processor from Heat Systems Ultra-Sonics Inc. for 30 minutes to thoroughly disperse the silicon nitride powder. The final pH of the slurry was 1.87. The resulting slurry having about 30 w/o powder loading was spray dried at about 190° C. to 200° C. and at a feed rate of 200 ml/hr. with a Buchi 190 Mini Spray Dryer. The spray dried powder was than calcined at 450° C. for 21/2 hours to convert the nitrate to the oxide of yttrium.

In the case of AY6, 500 g of SN502 silicon nitride powder prepared as described above was combined with 2.5 g ammonium citrate, 377 ml of yttrium nitrate solution at a concentration equivalent to 86.8 g/l of Y₂ O₃, 460 ml of aluminum nitrate solution containing an equivalent to 24.0 g/l of Al₂ O₃, and 400 ml of 1N ammonium hydroxide solution. The mixture was set milled with Si₃ N₄ milling media for 16 hrs. The slurry was sifted through a 350 mesh screen. The density of the slurry is about 1.3 g/ml. The final pH of the slurry was 5.60. The resulting slurry having about 35% powder loading was spray dried at about 190° C. to 200° C. and at a feed rate of 2 l/hr. with a Anhydro Spray Dryer or at 200 ml/hr. with a Buchi 190 Mini Spray Dryer. The spray dried powder was then calcined at 500° C. for 21/2 hours to convert the nitrates to the oxides of yttrium and aluminum.

The surface area of the resultant chemically processed powder is in the range of 18.0 m² /g to 22.0 m² /g compared to those of a conventional ball milled material which is in the range of 12.0 m² /g to 13.0 m² /g.

Pellets prepared without binder: Small pellets, 1.25 cm diameter, weighing about 1 gram were pressed at 210 MNm⁻² (30,000 psig) in WC dies. The green density of the pellet so prepared was in the 50.0 to 55.0% theoretical density region which is generally lower than those of pellets pressed with stearic acid as binder.

Pellets with binder: The chemical processed spray dried powder was wet milled with 4 w/o stearic acid in a methanol/toluene mixture for 1 hr. then followed by drying in air at 50° C. to 60° C. to remove the solvent. The dried powder was sifted through a 200 mesh screen prior to die pressing 1.25 cm diameter pellets at 20,000 to 50,000 psig. The pellets so prepared have a density in the range of 66.5 to 68.0% of T.D. Oleic acid was also used as a binder.

Preparation of Billets in AY6 and PY6 Formulation:

Two forming techniques were used to prepare (13/4"×11/2"×1/4") billets, isostatic pressing and slip casting.

A. Isostatic pressing: 20 to 23 grams of the chemical processed spray dried AY6/PY6 powders with 4 w/o stearic acid was isostatically pressed in a rubber mold at 170 MNm⁻² (25,000 psi). The billet was then dried in a desicator, and the binder was removed by heating at 450° C. The green density of the resultant billet is usually in the 65.0 to 67.0% theoretical density region.

B. Slip-cast: The slip-casting technique involved suspending 20 g of the milled particles in 9 g of slip solution containing carbowax, ammonium hydroxide, and ammonium citrate followed by uniaxial deposition of these particles into a bed formed against a material permeable to the suspending medium. The technique is akin to a "colloidal filtration" process. The billet (13/4"×11/2"×1/4") was then dried in a desicator overnight (12 hrs.), then followed by a binder removal procedure. The green density of the resultant billet is from about 65.0 to about 67.0% of theoretical density.

Sintering

The sintering procedure used was to place a graphite boat containing the AY6/PY6 samples into a cold sintering furnace. The furnace was evacuated and filled with nitrogen three times to remove air. The furnace was raised to the selected isothermal sintering temperature at a rate of about 30° C. per minute with a nitrogen flow. To determine the sintering behavior, time zero was the point at which the isothermal hold temperature was reached. Temperatures were controlled with a microoptical pyrometer. Densities after sintering were determined by immersion in water. X-ray diffraction analysis was conducted with Cu-K alpha radiation.

Mechanical Property Evaluation:

Molulus of rupture bars were diamond sawed from the densified shapes and surface ground to a 320 grit finish. Most strength test bars were 0.05×0.10×1.0" dimension, and were tested at room temperature, 1000° C., 1200° C., and 1400° C., respectively. The resultant modulus of rupture data were analyzed with Weibull Statistics having a computer program which calculates the Weibull parameter (m) by least-squares fit.

EXAMPLES

Example 1--Refer to samples 1 to 4 in Table 1. Small pellets, 1.25 cm diameter, weight about 1.0 g, were pressed at 210 MNm⁻² (30,000 psi) in WC dies. The green densities of the pellets so prepared were about 51.0% to about 54.6% of theoretical density. The pellets were sintered to greater than 99% of theoretical density at 1760° C. using a 200 psi N₂ overpressure. The sintering schedule was: holding at 1400° C. for 1 hr. followed by holding at the sintering temperature for 2 hrs.

Example 2. Refer to Samples 5 to 16 in Table I. The chemical processed powder in AY6 formulation was milled with 4 w/o stearic acid prior to die press 1.25 cm in diameter pellets at 20,000 to 50,000 psig. The pellets so prepared have green density in the range of 66.5 to 68.0% of T.D. Binder removal procedure was then followed. The sintering schedule was the same as Example 1.

Example 3. Three isostatically pressed AY6 billets were prepared as previously described in section (A.-Isostatic pressing:). The billets were sintered at 1765° C., with about 43 psi to about 46 psi N₂ overpressure. The sintering schedule was as follows: 1 hr. holding at 1400° C. and 3 hrs. holding at the sintering temperature. The results are tabulated as follows:

Sintering Conditions

    ______________________________________                                               Temp.    N.sub.2 Pressure                                                      °C.                                                                              (psi) at           Sintered                                                                              %                                     *Sam- Temper-  Sintering  Forming Density                                                                               Weight                                ple   ature    Temp.      Technique                                                                              (% T.D.)                                                                              Loss                                  ______________________________________                                         1     1765     43         isopressed                                                                             99.4   3.12                                  2     1765     43         isopressed                                                                             99.1   3.25                                  3     1765     46         isopressed                                                                             99.8   2.88                                  ______________________________________                                          *SN502 powder lot SN66                                                   

Example 4. One slip-cast AY6 billet was prepared as previously described in section B. Slip-cast. The chemical processed powder was premilled overnight prior to preparing the suspension. This billet was sintered at 1765° C. using a 43 psi N₂ overpressure. The sintered density was 99.73% of theoretical density (based on AY6 3.262 g/ml). (SN502 powder lot SN66)

                                      TABLE I                                      __________________________________________________________________________     SINTERING STUDIES OF SPRAY DRIED CHEMICAL PROCESSED AY6 PELLETS                (All SN502 lot 02-24)                                                                           Sintering Conditions                                                                 N.sub.2 Pressure at                                     Sample  Density of Green                                                                              Sintering Temp.                                                                        Sintering Density                               No.     Pellets (% T.D.)                                                                        Temp (°C.)                                                                    (psi)   % of T.D.                                                                               X-ray Diffraction                      __________________________________________________________________________     Example 1:                                                                      1      *52.2    1860  200     99.8     Single phase β-S.sub.3                                                    N.sub.4                                 2      *55.7    1860  200     99.9                                             3      *51.0    1760  200     98.5     Single phase β-Si.sub.3                                                   N.sub.4                                 4      *54.6    1760  200     98.7                                            Example 2:                                                                      5      67.1     1760  200     98.7                                             6      66.7     1735  200     99.0     Single phase β-Si.sub.3                                                   N.sub.4                                 7      68.0     1735  200     98.8                                             8 ball 66.8     1735  200     96.5                                            milled control                                                                 sample                                                                          9      66.5     1720  200     99.3     Major phase β-Si.sub.3                                                    N.sub.4                                                                        detectable amount of                                                           α-Si.sub.3 N.sub.4               10      67.5     1710  200     99.4     Major phase of β-Si.sub.3                                                 N.sub. 4                                                                       detectable amount Si.sub.2                                                     N.sub.2 O                              11 ball 67.1     1710  200     95.8     Major phase β-Si.sub.3                                                    N.sub.4                                                                        weak phase Si.sub.2 N.sub.2 O          milled control                                                                 sample                                                                         12      66.8     1750   60     99.5                                            13      67.1     1750   60     98.9                                            14 ball 67.0     1750   60     97.5                                            milled control                                                                 sample                                                                         15      66.5     1740   40     98.1                                            16 ball 67.0     1740   40     97.0                                            milled control                                                                 sample                                                                         __________________________________________________________________________

Example 5. Three slip-cast AY6 billets were prepared. These billets were sintered at 1765° C., using a 12 psi N₂ overpressure. The sintered densities were greater than 99% of theoretical density.

    ______________________________________                                                                         Sintered                                       Sample                                                                               Temperature      Overpressure                                                                            Density                                                                               % Weight                                No.   °C. N.sub.2                                                                              (psi)    % T.D. Loss                                    ______________________________________                                         1.    1765       12    slip-cast                                                                               99.6   2.42                                    2.    1765       12    slip-cast                                                                               99.3   2.18                                    3.    1765       12    slip-cast                                                                               99.2   2.14                                    4.    1765       12    slip-cast                                                                               98.7   1.79                                    ______________________________________                                          control ballmilled AY6                                                   

Example 6. Two slip-cast AY6 billets were sintered at 1690° C. using a 12 psi N₂ overpressure. The sintered density was in the 98.0-98.4% region of theoretical density.

    ______________________________________                                         Sample Sintering   Sintered    XRD                                             No.    Conditions  Density % T.D.                                                                             (X-ray diffraction)                             ______________________________________                                         12 psi 1690°                                                                               98.4%       beta-Si.sub.3 N.sub.4                           12 psi 1690°                                                                   98.0%       beta-Si.sub.3 N.sub.4                                       ______________________________________                                    

Example 7. One slip-cast AY6 billet was sintered at 1690° C., ambient pressure with continuous N₂ flow. The sintered density was 98.0% of theoretical density. XRD indicates that a complete alpha to beta-Si₃ N₄ phase transition was achieved.

Example 8. Two slip-cast PY6 billets (6.0 w/o Y₂ O₃) were sintered at 1850° C. using from 55 to 60 psi N₂ overpressure. The sintered densities were from 97.0% to 98.2% of theoretical density.

    ______________________________________                                         Sample Sintering      Sintered                                                 No.    Conditions     Density % T.D.                                                                             XRD                                          ______________________________________                                         1.     1850° C. - 55 psi N.sub.2                                                              98.2%       Beta-Si.sub.3 N.sub.4                        2.     1850° C. - 60 psi N.sub.2                                                              97.0%       Beta-Si.sub.3 N.sub.4                        ______________________________________                                    

Example 9. Mechanical test data for AY6 billets sintered at 1755° C. using 12 psi N₂ overpressure, and sintered at 1690° C. using ambient pressure are tabulated as follows:

    __________________________________________________________________________     Sintering     Sintered                                                                               Modulus of Rupture (Ksi)                                 Material                                                                            Conditions                                                                              Density % T.D.                                                                         Ambient                                                                             1000° C.                                                                     1200° C.                                __________________________________________________________________________     AY6  1765° C. - 12 psi                                                                99.5-99.8                                                                              117  101  63                                                  N.sub.2 Overpressure                                                      AY6  1690° C. - ambient                                                               98.0-98.4                                                                              107  101  63                                                  pressure                                                                  __________________________________________________________________________

Example 10. Mechanical test data for PY6 billets sintered at 1850° C. using 55 psi-60 psi N₂ overpressure are summarized as follows:

    __________________________________________________________________________     Sintering     Sintered                                                                               Modulus of Rupture (ksi)                                 Material                                                                            Conditions                                                                              Density % T.D.                                                                         Ambient                                                                             1000° C.                                                                     1200° C.                                __________________________________________________________________________     60-55 psi°                                                                            97.0-98.2                                                                              100  67   65                                                  N.sub.2 pressure                                                          __________________________________________________________________________

Charts A. and B. illustrate the sintering study of pellets of chemically processed pan-dried AY6 material vs. control-conventional ball-milled powders.

    ______________________________________                                                                Chemically                                                                     Processed AY6                                                                               Control                                                           Sintered     Sintered                                   Temperature                                                                              N.sub.2 Overpressure                                                                        Density      Density                                    ______________________________________                                         A. Setter Powder - SN502 + 0.5 w/o Al.sub.2 O.sub.3                            1850°                                                                             200     psi*     99.8       99.2                                     1810°                                                                             200     psi      97.2       96.6                                     1760°                                                                             200     psi      97.4       97.0                                     1700°                                                                             200     psi      97.2       95.8                                     1815°                                                                             60      psi**    99.4       99.4                                     1725°                                                                             60      psi      93.4       89.4                                     1710.sup. 60      psi      91.8       87.1                                     B. Setter Powder - AY6 + 1.5 w/o Y.sub.2 O.sub.3                               1850°                                                                             200     psi*     99.1       98.9                                     1725°                                                                             200     psi      97.9       96.5                                     1825°                                                                             60      psi**    98.1       98.1                                     1725°                                                                             60      psi      94.4       90.3                                     ______________________________________                                          *SN502-02-24 was used at 200 psi N.sub.2 overpressure sintering study.         **SN50202-37 was used at 60 psi N.sub.2 overpressure sintering study.          The mechanical strength of chemically processed (pan dried AY6) billet vs      billet prepared from conventional ballmilled powders is illustrated:     

The powder lot SN502-02-24.

Slip-casting was the consolidation technique.

The sintering condition was: 1850° C. using 200 psi N₂ overpressure. Two billets were sintered in the same boat.

    ______________________________________                                                     Modulus of Rupture                                                 Sample        RT         1000° C.                                                                          1200° C.                             ______________________________________                                         Chemically processed                                                                         110 ksi    95. ksi   64 ksi                                      pan dried AY6 (101.5-122)                                                                               (92-98)   (55-70)                                     Control AY6   90         86        53                                                          (84-101) (80-95)   (52-55)                                     ______________________________________                                    

The chemically processed AY6 (Spray Dried) billets vs. billets prepared from conventional ball-milled powders is illustrated:

The sintering conditions were: 1765° C. using 40-45 psi N₂ overpressure, holding at 1400° C. for 1 hr. and holding at the sintering temperature for 3 hrs.

The powder lot was: SN60-63, and the consolidation technique used was isostatical pressing.

    ______________________________________                                                  Sintered                                                              Sample   Density % T.D.                                                                              MOR at ambient                                                                               Weirbull                                   ______________________________________                                         Chemically                                                                              99.0         114 Ksi       13                                         processed             (102-127, 9 bars)                                        Control  98.5          96 Ksi        6                                                               (67-110, 10 bars)                                        ______________________________________                                    

                  TABLE II                                                         ______________________________________                                         Sintering Study of Chemically Processed Spray Dried AY6                        ______________________________________                                         Billets                                                                                                              Sintered                                 Sam- Powder     Sintering    Consolidation                                                                           Density                                  ple  Lot        Conditions   Technique                                                                               (% T.D.)                                 ______________________________________                                         1.   SN66       1 hr. 1400° C.,                                                                      isopressed                                                                              99.4                                                     4 hrs. at 1765° C.,                                                     40-45 psi N.sub.2 over-                                                        pressure                                                       2.   SN66       1 hr. 1400° C.,                                                                      isopressed                                                                              99.1                                                     4 hrs. at 1765° C.,                                                     40-45 psi N.sub.2 over-                                                        pressure                                                       3.   SN66       1 hr. 1400° C.,                                                                      isopressed                                                                              99.8                                                     4 hrs. at 1765° C.,                                                     40-45 psi N.sub.2 over-                                                        pressure                                                       4.   SN66       1 hr. 1400° C.,                                                                      slip-cast                                                                               99.7                                                     4 hrs. at 1765° C.,                                                     40-45 psi N.sub.2 over-                                                        pressure                                                       5.   SN60-63    1 hr. 1400° C.,                                                                      slip-cast                                                                               99.6                                                     4 hrs. at 1765° C.,                                                     12 psi N.sub.2 over-                                                           pressure                                                       6.   SN66       1 hr. 1400° C.,                                                                      slip-cast                                                                               98.7                                          Control AY6                                                                               4 hrs. at 1765° C.,                                                     12 psi N.sub.2 over-                                                           pressure                                                       7.   SN66       1 hr. 1400° C.,                                                                      slip-cast                                                                               99.3                                                     4 hrs. at 1765° C.,                                                     12 psi N.sub.2 over-                                                           pressure                                                       8.   SN66       1 hr. 1400° C.,                                                                      slip-cast                                                                               99.6                                                     4 hrs. at 1765° C.,                                                     12 psi N.sub.2 over-                                                           pressure                                                       ______________________________________                                         The Mechanical Strength of Chemically processed AY6                            billets sintered at 1765° C. using 12 psi N.sub.2 overpressure          follows:                                                                                    MOR Strength ksi                                                           Density                                                                        % T.D.    Ambient   1000° C.                                                                         1200° C.                          ______________________________________                                         Chemically                                                                              99.3      104       103      69                                       processed          (95-115,  (94-108, (65-77,                                  (SN66)             6 bars)   6 bars)  7 bars)                                  Control  98.7      99.4      91       68                                       AY6                (90-107,  (84-99,  (62-70,                                  (SN66)             4 bars)   4 bars)  3 bars)                                  Chemically                                                                              99.6      111       102      66                                       processed          (98-121,  (97-107, (65-68,                                  (SN60-63)          4 bars)   4 bars)  2 bars)                                  Chemically                                                                              99.5      118       103      63                                       processed          (109-124, (92-110, (62-64,                                  (SN60-63)          7 bars)   7 bars)  4 bars)                                  ______________________________________                                          The Mechanical Strength of Chemically processed AY6                           billets sintered at 1690° C. using 6-12 psi N.sub.2 overpressure        follows:                                                                                    MOR Strength ksi                                                  Sample   Density   Ambient   1000° C.                                                                         1200° C.                          ______________________________________                                         Chemically                                                                              98.0-98.4%                                                                               104 ksi   101 ksi  63 ksi                                   processed          (91-112,  (93-111, (57-66,                                  AY6 spray          13 bars)  6 bars)  6 bars)                                  dried                                                                          Control  95.1-96.% 97 ksi    95 ksi   60 ksi                                   AY6                (84-101,  (93-97,  (52-68,                                  spray dried        6 bars)   4 bars)  6 bars)                                  ______________________________________                                         The Mechanical Strength of Chemically processed PY6                            Billets sintered at 1850° C. using 55-60 psi N.sub.2 overpres-          sure follows:                                                                               MOR Strength ksi                                                  Sample   Density   Ambient   1000° C.                                                                         1200° C.                          ______________________________________                                         Chemically                                                                              97.0-98.2%                                                                               100 ksi   67 ksi   65 ksi                                   processed          (90-110 ksi                                                                              (64-70 ksi                                                                              (60- ksi                                 PY6                8 bars)   5 bars)  6 bars)                                  Control  98.1%     100 ksi   58 ksi   --                                       PY6                (90-108 ksi,                                                                             (51-65,                                                              5 bars)   3 bars)                                           ______________________________________                                    

Auger analysis of the individual coated particles substantiate that the silicon nitride particles have an essentially uniform coating of the sintering additives yttria and alumina.

    ______________________________________                                                   Atomic Percent                                                       Depth           C      N     O    AL    SI   Y                                 Angstrom                                                                               EV      272    379   510  1378  1606 1746                              ______________________________________                                         Pan Dried Sol. Gel Coated (AY6)                                                 0              7.9    10.4  48.2 7.5    9.3 16.7                               50             6.9    22.3  30.1 8.4   14.6 17.7                              100             8.5    24.7  26.0 4.9   14.1 21.8                              150             8.5    19.1  16.1 6.6   16.8 33.0                              200             10.3   24.2  17.3 6.8   13.0 28.4                              250             13.6   28.4  23.7 0.0   34.3  0.0                              Spray Dried Coated (AY6)                                                        0              8.8    11.3  45.6 3.6   12.8 18.0                               50             5.8    23.2  28.7 3.6   16.4 22.3                              100             9.5    14.4  19.2 7.2   21.9 27.9                              150             6.7    13.8  12.2 9.0   18.3 40.0                              200             9.9    25.2  17.8 4.8   20.8 21.4                              250             13.1   31.9  28.4 0.0   26.7  0.0                              ______________________________________                                    

While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A process for making a densified silicon nitride article comprising:Step 1--dispersing silicon nitride particles in a dispersion reagent to form a silicon nitride slurry; Step 2--adding a solution of sintering additives to said silicon nitride slurry to form a homogeneous slurry having a pH less than 7; Step 3--drying the product from step 2 to form a dried powder having an essentially uniform coating of said sintering additives on said silicon nitride particles; Step 4--converting said sintering additives of the product from step 3 to their corresponding oxides; Step 5--forming a silicon nitride article from the product of step 4; and Step 6--pressureless sintering the product from step 5 at a temperature less than 1700° C. to form a densified silicon nitride article having a density greater than 98 percent of theoretical density.
 2. A process in accordance with claim 1 wherein said dispersion agent comprises a solution having a high dielectric constant.
 3. A process in accordance with claim 1 wherein said dispersing comprises wet-ball milling.
 4. A process in accordance with claim 1 wherein said forming comprises pressing at 20,000 to 50,000 psig.
 5. A process for making a densified silicton nitride article comprisingStep 1--dispersing silicon nitride particles in a dispersion reagent to form a silicon nitride slurry; Step 2--adding a solution of sintering additives to said silicon nitride slurry to form a homogeneous slurry having a pH less than 7; Step 3--drying the product from step 2 to form a dried powder having an essentially uniform coating of said sintering additives on said silicon nitride particles; Step 4--Converting said sintering additives of the product from step 3 to their corresponding oxides; Step 5--forming a silicon nitride articles from the product of step 4; and Step 6--pressureless sintering the product from Step 5 at a temperature equal to or less than 1735° C. in nitrogen to form a densified silicon nitride article having a density greater than 99 percent of theoetical density.
 6. A process in accordance with claim 1 wherein said sintering additives are selected from the group consisting of soluble salts of yttrium, aluminum, magnesium, lanthanum, cerium, hafnium, zirconium, rare earths, and combinations thereof.
 7. A process in accordance with claim 6 wherein said soluble salts comprise nitrates.
 8. A process in accordance with claim 1 wherein said forming comprises isostatic pressing.
 9. A process in accordance with claim 1 wherein said forming comprises slip casting.
 10. A process in accordance with claim 5 wherein said temperature comprises a temperature equal to or less than 1860° C.
 11. A process in accordance with claim 1 wherein said dispersion agent comprises a one normal ammonium hydroxide solution containing ammonium citrate and ammonium acetate. 